In a dry cleaning machine used in cleaners and similar establishments, a vacuum distillation apparatus for purifying a solvent is employed to reuse a solvent (silicon oil, petroleum solvent and the like) contaminated as a result of cleaning laundry. Currently, such a distillation apparatus generally uses a solvent vapor condensation method in which a spiral pipe or similar pipe having a large surface area is disposed within a container filled with a solvent vapor, and water that has been cooled by a chiller or cooling tower is passed through the pipe to condense the vapor (for example, refer to Patent Document 1). Another method uses the same system in a reverse way: the container with the spiral pipe disposed therein is filled with cool water, and a solvent vapor is passed through the pipe to condense the solvent by heat exchange. These methods both use water as a medium for the heat exchange with the solvent vapor.
Water is easy to use as a cooling medium because it is higher in heat conductivity than oil and harmless to the human body. However, it also has various problems, such as the formation of rust or scale due to a long-term use, the freezing of water in cold climates, and the cooling water's temperature being unstable due to the influence of ambient temperature in the case of a cooling tower. There are also many inconveniences relating to maintenance. Given these factors, a distillation apparatus that does not use cooling water has been strongly demanded.
The aforementioned distillation apparatus includes a buffer tank for storing the solvent. The solvent in this tank is suctioned or pressurized by a pump to be circulated through an ejector, whereby the distillation still is evacuated. Since this pump circulation causes the solvent to generate heat, it is necessary to additionally dispose a pipe within the buffer tank and pass cooling water through this pipe to cool the solvent in the buffer tank. This means that a cooling container for condensing the solvent vapor must be provided separately from the buffer tank, which impedes the downsizing of the apparatus.
To solve this problem, a distillation apparatus disclosed in Patent Document 2 has a pair of parallel spiral pipes disposed within a buffer tank filled with a solvent, with the cooling water passing through one pipe and a solvent vapor through the other; the water is intended for cooling the solvent in the buffer tank, whereas the cooled solvent acts as a coolant for the solvent vapor. This configuration is advantageous with regards to downsizing because it enables the buffer tank and the cooling container for condensation to be consolidated into a single unit and also requires only a single system of a cooling-water supply line. However, there is also the problem that the temperature of the solvent in the buffer tank is higher than that of the cooling water and hence it is difficult to ensure a large temperature difference between the solvent and the solvent vapor, which leads to inadequate cooling of the solvent vapor and inappropriate condensation thereof. Another problem is that a large capacity buffer tank is required because the solvent in the buffer tank needs to have a large heat capacity.
Patent Document 1: Japanese Unexamined Patent Application Publication No. H7-289788
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2006-141546